"We urge North Korea to refrain from provocative actions that aggravate tensions and instead focus on fulfilling its international obligations and commitments," including UN resolutions, said State Department spokeswoman Jen Psaki.

The North must "exercise restraint and take steps to improve its relations with its neighbours," she added.

It is not unusual for North Korea to carry out such tests, which often go unreported by South Korea.

But South Korea said the Scud firings were of particular concern.

- 'Show of force' -

"We believe that the North is testing various ballistic missiles with various ranges as a show of force to threaten us," the defence ministry spokesman Kim said.

Last year the drills coincided with a sharp and unusually protracted surge in military tensions that saw North Korea issuing apocalyptic threats of pre-emptive nuclear strikes.

By contrast, this year's drills began as relations between Seoul and Pyongyang were enjoying something of a thaw.

They overlapped with the end of the first reunion for more than three years of families divided by the Korean War -- an event that raised hopes of greater cross-border cooperation.

Pyongyang had initially insisted that the joint exercises be postponed until after the reunions finished. But Seoul refused and -- in a rare concession -- the North allowed the family gatherings on its territory to go ahead as scheduled.

Most analysts believe the missile tests reflect Pyongyang's need to flex its muscles in the wake of the reunion compromise.

Last week also saw an incursion by a North Korean patrol boat across the disputed Yellow Sea border that has been the scene of brief but bloody naval clashes in the past.

No shots were fired and the vessel retreated to its side of the boundary after repeated warnings from the South Korean navy.

From Monday, South Korea launched a two-day live-fire exercise involving an army artillery, naval ships and jet fighters to test their readiness against a North Korean incursion off the east coast, the South's defence ministry said.